Containers, such as composite or metal containers, generally include a container body and metal ends which are joined together by a process referred to as double seaming. A double seam refers to the closure formed by interlocking and compressing an end portion of the metal end which has been preformed with an outer curl and the container body which has been preformed with an outer flange. The resulting double seam has a double lock profile defined by a body hook and a cover hook. The term "cover hook" is used to define that part of the double seam formed from the curl of the metal end. "Body hook" defines the portion of the flange of the container body that is turned down in the formation of the double seam. The first seaming operation of the double seaming process refers to the operation in which the curl of the metal end is tucked under the flange of the container body to form the cover hook and body hook. The second seaming operation refers to the finishing operation wherein the hooks formed in the first operation are rolled tightly against each other. To form a double seam, a rotating seaming chuck and a spring loaded base plate hold the metal end and container body together while first and second operation seaming rolls are cam sequenced in and out to form the double seam.
Such double seaming processes have been employed with metal cans. While the current technology is effective with metal cans, the technology, prior to this invention, has presented new challenges when sealing composite containers. This is due to many difficulties including wrinkling of the metal can end which commonly occurs in the double seaming operation. When such wrinkles (or "teeth") occur in the seam of a metal can, they may simply be ironed out, such as during the second seaming operation, without affecting the integrity of the metal body or the metal end. The resulting cans are therefore effectively sealed and the seam is hermetic. It has been established that the amount of wrinkling is a function of the metal thickness wherein wrinkle formation increases as the metal thickness, i.e., basis weight, decreases.
U.S. Pat. No. 5,595,322 to Kramer is an example of a metal can having a double seam joining the metal end and metal container body. A hermetic seal results because the wrinkles which are formed within the metal end during the curling step or the first seaming operation are ironed out during the second seaming operation. The existence of the wrinkles prior to being ironed out does not affect the integrity of the can body because it, too, is metal.
When this technology is applied to composite cans, however, several problems occur. A composite container may include a combination of compressible foil, paper and plastic wherein the foil layer may form the liner layer. The resulting seam is formed by a seaming process to hermetically seal the composite container body to the metallic end. The problems associated with composite containers are numerous. First, when wrinkling of the metal end occurs in the double seam, it often penetrates the composite can thereby destroying its liner layer rendering the composite can not hermetic. Second, the wrinkles cannot be easily ironed out from composite cans which often include a paper layer.
Double seams have been employed with composite containers wherein the containers are intentionally not hermetic such as are used in U.S. Pat. No. 5,005,728 to Mazurek et al. Wrinkling of the end occurs in these cans but this is desirable because the wrinkles actually assist in rendering these cans not hermetic. It is intended that these cans permit gases to escape, such as may occur during the proofing of packaged dough products. Additionally, wrinkles are encouraged because they assist in gripping and maintaining the end on the composite body of those cans.